my $thread_term   :shared = 0;
my $threads_init  :shared = 0;
my $file_read_sem : shared;                                                              ### Add this

...

   WORKER: while ( !$thread_term && defined ( my $file = $work_queue->dequeue ) )
   {
     {                                                                                   ### Add this
         lock $file_read_sem;                                                            ### Add this
         open my $fh, '<', $file or do { lock $d_counter; $d_counter++; next WORKER };

         my $data = <$fh>;

         close $fh;
      }                                                                                  ### Add this